The Origin of the Diversity of Type I[CLC]a[/CLC] Supernovae and the Environmental Effects

Observations suggest that the properties of Type Ia supernovae (SNe Ia) maydepend on environmental characteristics, such as morphology, metallicity, andage of host galaxies. The influence of these environmental properties on theresulting SNe Ia is studied in this paper. First it is shown that the carbonmass fraction X(C) in the C+O white dwarf SN Ia progenitors tends to be smallerfor lower metallicity and older the binary system age. It is then suggestedthat the variation of X(C) causes the diversity in the brightness of SNe Ia: asmaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of theturbulent flame are necessary to confirm this relation. Our model for the SN Iaprogenitors then predicts that when the progenitors belong to an olderpopulation or to a low metallicity environment, the number of bright SNe Ia isreduced, so that the variation in brightness among the SNe Ia is also smaller.Thus our model can explain why the mean SN Ia brightness and its dispersiondepend on the morphology of the host galaxies and on the distance of the SNfrom the center of the galaxy. It is further predicted that at higher redshift(z >~ 1) both the the mean brightness of SNe Ia and its variation should besmaller in spiral galaxies than in elliptical galaxies. These variations arewithin the range observed in nearby SNe Ia. In so far as the variation in X(C)is the most important cause for the diversity among SNe Ia, the light curveshape method currently used to determine the absolute magnitude of SNe Ia canbe applied also to high redshift SNe Ia.Comment: 4 pages, 3 figures, Accepted for publication in ApJ Letter